Need help/suggestions for domain-less login

I am setting up an ISPConfig3 system to replace a manually-configured Postfix/Courier system for ISP-style e-mail access. My ISPConfig install went quite smoothly, but I am left with one problematic issue. ISPConfig e-mail boxes need to be logged into with a [email protected] style address, but this particular box (will only be serving one domain) needs to accept username-only (with appropriate-password, of course) logins for IMAP and POP.

Has anyone run into this before (seems likely to me), and is there any configuration advice on this? Any and all input is greatly appreciated.